Reflector

ABSTRACT

A reflector includes a frame over which is stretched a flexible sheet of reflective material to define a curved reflecting surface of pre-determined geometry. The frame defines arcuate sections portions of which support the sheet and lie on the surface of pre-determined geometry. At least one of the arcuate sections is deformable to displace the respective sheet supporting portion and thereby adjust the tension in the flexible sheet.

This application is a 371 of PCT/GB96/02596 filed Oct. 23, 1996.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a reflector comprising a flexible sheetof reflective material stretched over a frame to define a curvedreflecting surface. Particularly, but not exclusively, the inventionrelates to the shaping of a thin reflective film in the construction ofa part spherical, spheroidal or toroidal mirror.

2. Related Art

Reflectors comprising a sheet of reflective material formed intopart-spherical and other curved configurations are known. For instance,flight simulators and other optical training systems typicallyincorporate spherical mirrors which are conventionally produced bystretching a thin reflective film (typically a metallized plasticsmaterial) over a support frame which defines the necessary curvature,and then partially evacuating air from one side of the film.

In more detail, conventional methods for constructing such sphericalmirrors comprise cutting a sheet of the reflective film into anappropriate shape and then stretching the film over the walls of a rigidsupport frame the edges of which have the appropriate curvature (i.e.lie on the desired curved surface). The edges of the film are sealedwith respect to the frame so that the film and the frame together form asubstantially air-tight chamber. The shaping of the mirror is thencompleted by partially evacuating the chamber so that the thin filmassumes the desired spherical configuration, i.e. the film is suckedinto the chamber forming a surface corresponding to a segment of asphere. The same method can be used to construct mirrors with othercurved geometries.

A problem with such methods of construction is that distortion can beintroduced in regions towards the edges of the reflective film resultingfrom the need to support the film such that it has the necessarycurvature in two orthogonal directions.

For instance, in the construction of spherical mirrors for use in theoff-axis optical systems of flight simulators, the film of flexiblematerial is initially cut into the shape of a blank for a frustum of acone, having two relatively long side edges and two relatively short endedges. When mounting the film to its support frame, the relatively longedges are fixed to side walls of the frame first so that they arewrinkle free but without introducing significant tension into the film.The film will readily accept the curvature imposed by securing the twoside edges of the film to the side walls of the mounting frame. Indeedthe film is typically obtained from a roll and so will already have atendency to curl in one direction.

The distortion referred to above is a result of the need to secure endedges of the film to end walls of the support frame, the edges of whichcurve in a direction substantially orthogonal to the direction ofcurvature of the edges of the side walls of the support frame. That is,initially (once the side edges of the film have been secured to thesupport frame) the end edges of the film will form a chord extendingbetween the side walls of the support frame. To secure the end edges ofthe film to the support frame the film must be stretched onto its endwalls. This stretching process can introduce areas of tension and/orlooseness into the film which result in the distortion of the film atits edges once the chamber has been evacuated to produce the desiredmirror shape. Clearly any such distortion will have an adverse effect onthe performance of the mirror.

To obviate this problem, spherical mirrors are often made to a sizelarger than is actually required and the distorted regions are simplymasked off or otherwise hidden.

An alternative solution to this problem is proposed in U.S. Pat. No.4,592,717. This discloses apparatus comprising a support frame generallyas described above but wherein the end edges of the film are mounted tothe end walls of the support frame using flexible stretching members.The stretching members are initially straight (following the chordsdefined by the end edges of the film as mentioned above) but aresubsequently bent to match the curvature of the edges of the end wallsof the support frame. As the stretching members are bent they force theend edges of the reflective film onto the end walls of the frame towhich they are then clamped. According to this U.S. patent, theconfiguration and positioning of the stretching members is such that asthey are bent they stretch the film slightly beyond its yield point andin such a way as to eliminate the irregularities in tension that wouldotherwise cause distortion in the film.

In another known method for forming curved mirrors, a support framesubstantially as described above is provided with extensions to its sidewalls which extend beyond the frame end walls and are closed off bysecondary end walls. The film is cut larger than required and isinitially mounted to the frame with its end edges secured to thesecondary end walls so that the film extends beyond the frame end wallswhich define the desired size and curvature of the mirror. A curvedformer, having the same profile as the frame end walls, is then used toforce (and thus stretch) the film onto the frame end walls and clamp itthereto. The extension sections can be either left in place or removedif desired.

SUMMARY OF THE INVENTION

It is an object of the present invention to obviate or mitigate theproblems discussed above.

According to a first aspect of the present invention there is provided areflector comprising a frame and a flexible sheet of reflective materialwhich is stretched over the frame to define a curved reflecting surfaceof predetermined geometry, the frame defining arcuate sections portionsof which support the sheet and lie on the surface of predeterminedgeometry, wherein at least one of the arcuate sections is deformable todisplace the respective sheet supporting portion and thereby adjust thetension in the flexible sheet.

With the present invention the (or each) deformable arcuate section can,for instance, be deflected with the reflector in situ to adjust thetension in the reflective sheet to improve the performance of thereflector. Moreover the (or each) deformable arcuate section of thesupport frame can be deflected to allow the flexible sheet to beattached thereto substantially without stretching the flexible sheet,and then subsequently moved to a position on the desired curved surface,stretching the flexible sheet into the required configuration in theprocess. Stretching the flexible sheet in this way after it has beenfixed to the frame is a convenient way of stretching the sheet into therequired shape whilst minimising irregularities in the tension in thesheet to reduce distortion.

Thus, according to a second aspect of the present invention there isprovided a method of forming a reflector with a curved surface ofpredetermined geometry, the method comprising stretching a flexiblesheet of reflective material over a frame which defines arcuate sectionsportions of which support the sheet and lie on the curved surface,wherein the respective sheet supporting portion of at least one of thearcuate sections is initially displaced to allow the reflective sheet tobe attached thereto substantially without stretching the sheet in thedirection of the curved surface, whereafter the displaced sheetsupporting portion is moved to a position on the curved surface therebystretching the sheet as it moves.

The method may include the additional steps of partially evacuating thechamber formed by the frame with the flexible sheet mounted thereto todraw the flexible sheet into the desired curved surface and thendisplacing the sheet supporting portion of at least one of said arcuatesections to thereby adjust the tension in the flexible sheet.

In a preferred embodiment of the invention, the frame defines four ofsaid arcuate sections at least two of which are deformable. All fourarcuate sections may be deformable in which case the support framepreferably comprises a lattice structure which supports said arcuatesections. Such a lattice structure may have a relatively light weightconstruction which is advantageous where the reflector is intended foruse in applications in which manoeuvrability is important.

The reflector may include means for displacing the sheet supportingportion of the or each deformable arcuate section. Such displacing meansmay, for instance, comprise a relatively rigid member which extendsadjacent a respective deformable arcuate section, and a plurality ofjacking means which extend between said relatively rigid member and thedeformable arcuate section, such that suitable operation of the jackingmeans displaces the sheet supporting portion of the deformable arcuatemember towards or away from the relatively rigid member.

The jacking means may, for example, comprise jacking screws which extendthrough the respective deformable arcuate section and engage saidrelatively rigid member.

The reflecting surface may have any desired curved geometry and maycurve in two orthogonal directions, for instance forming part of thesurface of a sphere, a spheroid, or a toroid.

BRIEF DESCRIPTION OF THE DRAWINGS

Specific embodiments of the present invention will now be described, byway of example only, with reference to the accompanying drawings, inwhich:

FIGS. 1 and 1a are is a perspective view of a thin film mirror supportframe in accordance with the present invention;

FIGS. 2 and 3 are side and end elevations of the support frame of FIG. 1looking in the directions of arrows A and B respectively;

FIG. 4 is a plan view of a sheet of thin reflective film cut formounting to the support frame of FIG. 1;

FIG. 5 is a perspective view showing the thin film of FIG. 4 partiallymounted to the support frame of FIG. 1;

FIG. 6 illustrates one end of the support frame of FIG. 1 configured formounting the thin film thereto; and

FIG. 7 illustrates a support frame in accordance with a secondembodiment of the present invention.

DETAILED DESCRIPTION OF EXEMPLARY EMBODIMENTS

Referring to the drawings, the embodiment of the invention illustratedin FIGS. 1 to 6 is a part spherical mirror intended for use in an offaxis collimation system of a flight simulator (or similar trainer) andis thus shaped accordingly. The mirror comprises a support frame,indicated generally by the reference 1, to which is mounted a thinflexible reflective film 2 of a metallized plastics material.

The support frame I comprises relatively long side support walls 3 and 4connected at their ends by relatively short end support walls 5 and 6.The upper edges 3a, 4a, 5a and 6a respectively of the side walls 3, 4, 5and 6 are arcuate and all lie on the desired spherical surface. i.e. thearcuate edges 3a, 4a, 5a and 6a each have the same centre and radius ofcurvature. Because the mirror is intended for use in an off-axiscollimation system the side wall edges 3a and 4a define sections oflatitudinal small circles of the spherical surface and the end walledges 5a and 6a define sections of longitudinal great circles.

The side walls 3 and 4 are constructed from a relatively rigid materialwhereas the end walls 5 and 6 are constructed from a deformablematerial. Rigid U-sectioned cross members 7 extend between the sidewalls 3 and 4 adjacent the end walls 5 and 6 respectively. Each of theend walls 5 and 6 is connected to a respective cross member 7 by aseries of jacking screws 8. Details of a jacking screw 8 are shown inthe FIG. 1a. In the illustrated mirror the jacking screws 8 areequi-spaced, it will however be appreciated that the spacing could vary.For instance, the jacking screws 8 could have reduced spacing towardsthe middle of the respective walls.

The arrangement is such that by suitable adjustment of the jackingscrews 8, the end walls 5 and 6 can be deflected towards or away fromthe respective cross member 7. Furthermore, the degree of deflection canbe independently adjusted at the site of each individual jacking screw8.

In preparation for mounting to the support frame 1, the reflective film2 is first cut to an appropriate shape, which in this case is that of ablank of a part frustrum of a cone as illustrated in FIG. 4. The sheetof reflective film 2 as illustrated has relatively long side edges 9 and10 and relatively short end edges 11 and 12. The length of the sideedges 9 and 10 corresponds to the circumferential length of the arcuateedges 3a and 4a of the frame side walls 3 and 4 respectively.

The reflective film 2 is mounted to the support frame 1 by firststretching it over the side walls 3 and 4 and securing its edges 9 and10 to the side wall edges 3a and 4a. This is readily done withoutintroducing wrinkles or any irregular tension into the reflective film 2which might cause distortions in the curvature of the surface. At thisstage, the edges 11 and 12 of the reflective film 2 extend between theside walls 3 and 4 of the support frame 1 along chordal lines extendingbetween the opposite ends of each edge 5a and 6a respectively (see FIG.5). To complete the mounting of the reflective film 2 to the supportframe 1, the edges 11 and 12 of the film 2 have to be secured to thearcuate edges 5a and 6a respectively. However, in forcing the edges 11and 12 of the film 2 towards the arcuate edges 5a and 6a, the edges 11and 12 tend to deflect inwards along the paths shown by the dotted lines13 and 14.

To take account of this deviation from the desired shape, the edges 5aand 6a of the deformable side walls 5 and 6 respectively are deflectedinwards to take up the path shown by the dotted lines 13 and 14 byappropriate adjustment of the jacking screws 8, as illustrated in FIG. 6(which shows one end only of the frame and with the film removed). Thisallows the end edges 11 and 12 to be secured to the support wall edges5a and 6a substantially without stretching the film. The side walls 5and 6 are then returned to their original position (which defines thedesired curvature of the mirror) again by appropriate adjustment of thejacking screws 8. Formation of the mirror is completed by partiallyevacuating the air from the chamber formed by the support frame 1 andreflective film 2.

The apparatus thus stretches the film as required after it has beenmounted to the end walls 5 and 6, and in such a way as to tension thefilm so that distortion is minimised in the completed mirror. Any errorsin the shape of the mirror through any initial inaccuracy in adjustmentof the support frame end walls 5 and 6 can be corrected once the mirroris in situ by further adjustment of the jacking screws 8 to adjust thetension in the film 2 whilst testing the performance of the mirror.

The present invention thus provides a mirror assembly which simplifiesthe process of stretching the reflective film over the supporting frameand allows for accurate adjustment of the tension in the film tominimise distortions near the edges of the mirror.

It will be appreciated that the shape and configuration of the supportframe 1 may vary widely depending upon the desired shape and curvatureof the mirror. For instance, the mirror could be part spheroidal or parttoroidal.

It will further be appreciated that details of the jacking screws 8 andof the cross members 7 may be varied, as may the exact number andpositioning of the jacking screws. For instance, the cross members 7could be curved to match the curvature of the end wall edges 5a and 6a.Similarly, it will be appreciated that a variety of means and methodscould be devised to deflect the walls 3 and 4.

Whereas in the embodiment described above, the side walls 3 and 4 arerelatively rigid, these may be replaced with flexible walls andcorresponding jacking assemblies similar to those of the end walls 5 and6. This would allow the tension in the reflective film 2 in the regionsof its longer edges 9 and 10 to be adjusted in situ to improve theperformance of the mirror. For instance, a second embodiment of theinvention which has flexible side walls 3 and 4 as well as flexible endwalls 5 and 6 is illustrated in FIG. 7. Because the tension in all fourwalls can be adjusted after the mirror film has been mounted to thesupport frame, the support frame can be constructed with a light-weightflexible construction.

Thus, referring to FIG. 7 (and using the same reference numerals asabove where appropriate) the illustrated support frame 1 comprises alattice structure of light-weight tubes 15 which themselves support thefour support walls 3, 4, 5 and 6. The corners of the frame must bemaintained in a fixed position and this is achieved by rigid cornerstays 16. Rigid longitudinal and latitudinal cross members 7 and 7a areprovided to receive jacking screws 8 from the end walls 5 and 6 and sidewalls 3 and 4 respectively. A light-weight flexible sheet material,similar to the mirror film, may be used to close the back of the supportframe 1 so that the required closed chamber is formed once the mirrorfilm 2 has been mounted to the support frame 1.

In use, when the mirror chamber is evacuated the flexible walls willtend to deflect inwards. The jacking screws can then be used to adjustthe walls to the correct position, under the evacuated conditions, togive the desired mirror shape with minimal distortion around its edges.When the mirror is not in use, and thus the pressure within the chamberincreases, the structure will relax but will once again assume thecorrect configuration when next used and the chamber is evacuated.

A light-weight support frame such as that illustrated in FIG. 7 hassignificant advantages over heavier rigid conventional support frames inapplications where movement of the mirror is required, as is typical intraining systems.

It will be appreciated that the detailed structure of the supportlattice may be varied, as may the overall configuration of the mirrordepending upon its intended application.

It will also be understood that the present invention is not restrictedto the construction of optical mirrors but extends to other types ofreflector.

What is claimed is:
 1. A reflector comprising:a frame and a flexiblesheet of reflective material which is stretched over the frame to definea curved reflecting surface of predetermined geometry, the framedefining arcuate sections, portions of which support the sheet and lieon the surface of pre-determined geometry, wherein at least one of thearcuate sections is selectively deform to displace the respective sheetsupporting portion towards and away from the interior of the frame andthereby adjust the tension in the flexible sheet.
 2. A reflector as inclaim 1, wherein the curved reflecting surface curves in two orthogonaldirections.
 3. A reflector as in claim 1 wherein the frame defines fourof said arcuate sections at least two of which are deformable.
 4. Areflector as in claim 3, wherein all four arcuate sections aredeformable.
 5. A reflector as in claim 1 wherein said frame comprises alattice structure which supports said arcuate sections.
 6. A reflectoraccording to claim 1 wherein means are provided for displacing the sheetsupporting portion of said at least one deformable arcuate section.
 7. Areflector as in claim 6, wherein each means for displacing comprises:arelatively rigid member which extends adjacent a respective deformablearcuate section, and a plurality of jacking means which extend betweensaid relatively rigid member and the deformable arcuate section, suchthat operation of the jacking means displaces the sheet supportingportion of the deformable arcuate member towards or away from therelatively rigid member.
 8. A reflector as in claim 7 wherein eachjacking means comprises:a jacking screw which extends through therespective deformable arcuate section and engages said relatively rigidmember.
 9. Apparatus as in claim 1 wherein the curved surface is part ofa sphere, a spheroid, or a toroid.
 10. A method of forming a reflectorwith a curved surface of predetermined geometry, the methodcomprising:stretching a flexible sheet of reflective material over aframe which defines arcuate sections portions of which support the sheetand lie on the curved surface, initially displacing the respective sheetsupporting portion of at least one of the arcuate sections towards theinterior of the frame to allow the reflective sheet to be attachedthereto substantially without stretching the sheet in the direction ofthe curved surface, and thereafter moving the displaced sheet supportingportion to a position on the curved surface thereby stretching the sheetas it moves.
 11. A method as in claim 10, wherein the frame with saidflexible sheet mounted thereto together form a chamber and the methodcomprises the additional steps of:partially evacuating said chamber todraw the flexible sheet into the desired curved surface, and thendisplacing the sheet supporting portion of at least one of said arcuatesections to thereby adjust the tension in the flexible sheet.
 12. Amethod as in claim 10 or 11, wherein the curved surface curves in twoorthogonal directions.
 13. A reflector comprising:a frame having edgeswhich define arcuate convex sections on the circumference of apredetermined geometrical surface having two dimensions of curvature; aflexible sheet of reflective material stretched over said edges of theframe; and at least one of said edges being adjustable in acircumferential direction that is generally parallel to the stretchedsheet surface to thereby facilitate initial attachment of the sheetthereto and subsequent adjustment of tension in the stretched sheet. 14.A reflector as in claim 13 wherein:the frame has four outer edges andtwo opposing ones of said outer edges are adjustable in saidcircumferential direction.
 15. A reflector as in claim 13 wherein saidat least one adjustable edge includes:a plurality of spaced-apartadjustment fixtures distributed therealong to facilitate effectingdifferent magnitudes adjustment along the associated edge.
 16. Areflector as in claim 14 wherein each said adjustable edge includes:aplurality of spaced-apart adjustment fixtures distributed therealong tofacilitate effecting different magnitudes adjustment along theassociated edge.
 17. A reflector as in claim 15 wherein:two opposingedges of said frame are rigid and are interconnected by a rigidstructure to which said adjustment fixtures are adjustably connected.18. A method of forming a reflector, said method comprising:providing aframe having edges which define arcuate convex sections on thecircumference of a pre-determined geometrical surface having twodimensions of curvature; stretching a flexible sheet of reflectivematerial over said edges of the frame; and initially adjusting at leastone of said edges in a circumferential direction that is generallyparallel to the stretched sheet surface to thereby facilitate initialattachment of the sheet thereto; and subsequently adjusting tension inthe stretched sheet by further adjusting said at least one edge in acircumferential direction.
 19. A method as in claim 18 wherein:the framehas four outer edges and two opposing ones of said outer edges areadjusted in said circumferential direction.
 20. A method as in claim 18wherein said adjusting steps include:individually adjusting a pluralityof spaced-apart adjustment fixtures distributed along the adjustableedge to facilitate effecting different magnitudes adjustment along theassociated edge.
 21. A method as in claim 19 wherein said adjustingsteps include:individually adjusting a plurality of spaced-apartadjustment fixtures distributed along the adjustable edge to facilitateeffecting different magnitudes adjustment along the associated edge. 22.A method as in claim 20 wherein two opposing edges of said frame arerigid and are interconnected by a rigid structure to which saidadjustment fixtures are adjustably connected.